我剛剛說到哪兒了？
Where was I？
What was I saying?
I just had it!

怎么說來著？
It's on the tip of my tongue...
話就在我嘴邊啊！Tongue是舌頭，tip是尖端，tip of my tongue很生動，表示就要想起來，但吐不出來。
How shall I put it?
我該怎么說呢？
有時候沒想到怎么措辭，可以說這句，或者有時候別人問你問題，沒有直接給答案，需要理一下思緒，也可以說這個。
What's the word for it...
這個怎么說來著？旁邊如果有懂你的人就會幫你補上。
Now, let me see.
讓我想想。講到一半想不出下文可以說Now, let me see.　停下來想想，給自己的對話爭取一下時間。

思路被打斷，腦袋一片空白
Sorry I lost my train of thought.
train of thought 表示一連串的思考，比如
Please don't interrupt my train of thought when I'm writing.
我寫作的時候請不要打斷我的思路。
Sorry I lost my train of thought.
我剛剛一下子思路被打斷了。
Sorry I spaced out there for a moment.
space out: To lose concentration or become inattentive
使間隔開；變得昏昏沉沉
加寬行距或字距
I was supposed to meet her at 9:00, but I spaced out and didn't get there till 10:30.
我本來9點就要去見她，但是我不知道咋的走了神，10點半才到。
I completely spaced out during the lecture.
我聽講座的時候整個人都飄了。
I’m such a scatter brain.
我真是腦子亂的不行

我們剛剛在說啥？
I completely forgot what I was saying?
我完全忘記我剛在說啥
What on earth was I talking about?
我到底在說啥？
I must be loosing my mind, I forgot what we were talking about.
我可能是瘋了吧，我不記得我們在說啥。